The present invention relates to x-ray systems, and more particularly, to the alignment of the focal spot in an x-ray tube.
In a contemporary computed tomography system, an x-ray tube projects a fan-shaped beam which lies within the X-Y plane of a Cartesian coordinate system, termed the "imaging plane." The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. Attenuation measurements from all detectors are acquired separately to produce the transmission profile.
The tube and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. The set of x-ray attenuation measurements from the detector array at a given angle is referred to as a "view"; and a "scan" of the object is comprised of a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image representative of a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
The image reconstruction process relies on a very accurately positioned focal spot from which the fan-shaped x-ray beam emanates. The focal spot is the location on the x-ray tube anode which is struck by an electron beam emanating from a cathode. Misalignment of this focal spot by as little as 0.025 mm can result in sampling errors that reduce image resolution and produce image artifacts.
Perfect mechanical alignment of the x-ray tube focal spot is difficult to achieve in a commercial production setting and difficult to maintain in a clinical setting. Calibration and alignment procedures are used to position the x-ray tube focal spot during initial manufacture and during tube replacement in the field. These procedures are delicate and time consuming. In addition, such static focal spot alignment does not account for small displacements that can occur during the operation of the scanner due to thermally-related dimensional changes in the x-ray tube structures.